Diagnostic ultrasonic scanning systems commonly rotate an ultrasonic transducer back and forth through a prescribed angle (sector) while rapidly transmitting high frequency sound waves into a biological structure. Thereafter, the reflected waves are received and processed to generate a two dimensional cathode ray tube display of the biological morphology revealed by the intensity and origin of the echos received.
The reciprocal transducer motion required in such scanning must be (1) of stable and repeatable amplitude and (2) of substantially constant frequency. A variety of mechanisms have been employed to generate motion with the requisite features. Motor driven cam actuators, cranks and eccentrics as well as stepping motors have been used successfully in probes for such scanning devices, but the recent development of low cost, high speed scan converters which permit high resolution displays on inexpensive television monitors have imposed an additional constraint upon the scan head frequency. Since the frame rate used in television system is fixed precisely at 30 Hz in the U.S., echo data must be received and processed synchronously with this frequency if, as is normally required in diagnostic medical examinations, the display is to instantly reflect the probe application.
It is possible to employ A.C. synchronous motors to achieve the required scan frequency precision, but such A.C. motors are relatively large compared to the D.C. motors which have previously been used. Also, probes incorporating such A.C. motors are bulky and difficult to manipulate in diagnostic procedures: particularly in those examinations of relatively small organs such as the eye. Stepper motors can also be operated to provide a specified scan frequency, but their motion is not continuous. In high resolution ultrasonagraphy, the use of a discontinuous, stepped scan head motion can itself limit the resolution of signal processing circuitry that may be capable of revealing finer detail than the number of discrete motional steps permitted by the motor. Stepper motors are also larger than D.C. motors having the same power output and inevitably limit a reduction in probe size.
Because none of the existing probe designs could satisfy the need for a small hand held probe operating at a precisely predetermined frequency with a well controlled amplitude, a new mechanism was developed that has the required features with not intrinsic limit to miniaturization.